1. Technical Field
The present invention relates to an asynchronous transfer mode (ATM) switching system suitable for multicast data communication, and more particularly to an apparatus and method for processing ATM cells for use in a subscriber board of the ATM switching system.
2. Related Art
Generally, ATM is a specific packet-oriented transfer mode using an asynchronous time division multiplexing technique where both line switching and packet switching are unified and many pieces of information such as voice, data, and pictures are organized in fixed-sized blocks, called cells. ATM can operate as both a packet exchange system and a circuit exchange system in one transmission mode for efficiently embodying both high speed and wide band communication networks. For this reason, ATM is a target transfer mode solution for implementing a broadband integrated services digital network (B-ISDN) because it offers an effective information channel management based upon virtual path and virtual channel identifiers ("VPI" and "VCI") placed in a header of each cell which facilitates the use of a multiplex, de-multiplex and exchange of digital information, and serves to decrease the costs of broad-band communication network facilities because of its flexibility in a bandwidth distribution.
Typically, data communication will be in the form of singlecast, which is a point-to-point connection. An example of singlecast data communication is a normal two-party telephone call. The voice of one caller is transferred to the other party and no other. In many broadband integrated services digital networks (B-ISDN), there is increasingly a need for multicast data communication, a point-to-multipoint connection, in which the same data to sent simultaneously to selected stations in the network. The extreme of multicast is broadcast, in which all stations receive the same data, in the same way that television and radio transmissions are broadcasts. An example of multicast data communication is a video conference in a CATV system.
In the case of a point-to-point ATM connection, cells are relayed from the source to the target without being duplicated or copied, while in a multicast connection, cells from the source are duplicated within the network and then separately routed to each destination in turn. In ATM multicast as disclosed, for example, in U.S. Pat. No. 5,202,885 for ATM Exchange With Copying Capability issued to Schrodi et al., U.S. Pat. No. 5,229,991 for Packet Switch With Broadcasting Capability For ATM Networks issued to Turner, U.S. Pat. No. 5,305,311 for Copy Network Providing Multicast Capabilities In A Broadband ISDN Fast Packet Switch Suitable For Use In A Local Area Network issued to Lyles, U.S. Pat. No. 5,410,540 for Shared-Buffer-Type ATM Switch Having Copy Function And Copy Method Thereof issued to Schrodi et al., U.S. Pat. No. 5,435,893 for ATM Cell Switch Suitable For Multicast Switching issued to Barnett, U.S. Pat. No. 5,459,724 for ATM Switching Arrangement issued to Jeffrey et al., and U.S. Pat. No. 5,541,915 for Method For Point-To-Multipoint Connections In Self-Routing ATM Switching Networks issued to Storm, the cells must not only be duplicated in the ATM switch, but must also have the correct VPI/VCI assigned to the header for each copy of the cell. The VPI/VCIs will be different for each cell, since the destinations will all be different. A cell head translation apparatus is typically used to assign different VPI/VCI with respect to the respective copied ATM cell. The cell header translation apparatus copies the cell as many as the number of the output points by means of a switching circuit for routing the cells to the output ports, and determines the output ports based on a copy identification number and the original VPI/VCI. Then, the cell header translation apparatus translates a header of the cells into the VPI/VCI required at the output ports, and transfers the cells with the translated header to the output ports. However, in case that a plurality of subscribers (medium and low-speed subscribers) each being smaller than the maximum capacity of the switch are connected to one switch port, such a copy network may not provide the point-to-multipoint communication service.
For example, in case that the ATM switching system has a subscriber network with a switch port to which 64 E1-class subscribers SAE1 or 3 DS3-class subscribers SAD3 are connected, ATM cells copied from one signal source should be transferred to the output ports of the same switch. In such a situation, since two or more cells can not be transferred at the same time to the same output port, only one of the cells is selectively transferred and the remaining cells are fed back to be buffered. In case of 64 E1-class subscribers SAE1, only one of the cells is transferred and the remaining 63 cells should be buffered simultaneously, thereby unavoidably disusing the remaining cells. Therefore, routing information based on a look-up using the respective headers should be necessarily provided in order to rout one of three cells in case of DS3-class connection, or one of 64 cells in case of E1-class connection to the designated subscriber ports.